1. Field of the Invention
This invention concerns a multiple transfer molding die used for molding a plurality of semiconductor devices. More particularly, this invention concerns a construction of a runner or a resin path between a pot and cavities in a multiple transfer molding die.
2. Description of the Prior Art
Conventionally, a multiple transfer molding die is used to mold a plurality of semiconductor devices.
FIGS. 1 and 2 are drawings of a conventional multiple transfer molding die. In the drawings, only one plunger and half of the group of cavities disposed around the plunger are shown for convenience of the description. FIG. 1 is a cross sectional view, and FIG. 2 is a plan view of the molding die.
In the drawings, numeral 1 designates an upper die portion, numeral 2 designates a lower die portion, numeral 3 is a pot, and numeral 4 is a plunger. Numeral 5 designates cavities. Numeral 8 shows a resin tablet put in the pot 3 to be transferred to the cavities by the press of the plunger 4. Numeral 11 designates a center block which includes the pot 3 and the plunger 4. Numeral 12 designates a cull runner, 13 designates a sub-runner and 14 designates a gate runner. Numeral 15 designates gates. A semiconductor chip 7 mounted on a lead frame 6 is disposed in the cavity 5. The lead frame 6 is cramped by the upper die 1 and the lower die 2, and the semiconductor chip 7 is electrically connected to the lead frame 6 by bonding wires 10.
In this construction, as is well known, when the plunger 4 is pushed up, the resin tablet 8 in gel condition is transferred to the cavity 5 through the cull runner 12, the sub-runner 13, the gate runner 14 and the gate 15.
In the conventional die, the sub-runner 13 is formed in parallel to the longitudinal axis of the lead frame 6. Thus, the path between the pot 3 and the cavity 5a becomes longer than that of between the pot 3 and the cavity 5b (see FIG. 2). Therefore, the mold resin reaches cavity 5b first, which causes a time lag with respect to when the other cavities are filled with resin.
The pressure is not applied to any cavity until all the cavities are filled with the resin. Thus, when the resin hardens rapidly, the hardening progresses in an inadequate condition in the cavity 5b, and sometimes voids are formed in the housing of the semiconductor devices. Even if no voids are formed, the somewhat hardened resin may cause a wire deformation or a wire cut in an extreme case.
Furthermore, to allow automated continuous molding, an appropriate margin for the molding time is required. For example, the transferring time of resin, namely the time from the beginning of the plunger push up to the beginning of the curing, varies about 2 seconds in an automated molding.